Due to their technological importance and social economic value, semiconductor photodetectors and image intensifiers have been studied and widely used for more than half a century. Listed below are references immediately relevant to this invention:
J.-P. Vilcot, V. Magnin, J. Van de Casteele, J. Harari, J.-P. Gouy, B. Bellini, D. Decoster, Workshop on High Performance Electron Devices for Microwave and Optoelectronic Applications, pp. 163–168, Nov. 24–25, 1997.
M. C. Brain, and D. R. Smith, “Phototransistors in digital optical communication systems”, IEEE Journal of Quantum Electronics, Vol. QE-19, No. 6, pp. 1139–1148, 1983.
Heinz Beneking, N. Grote, W. Roth, and M. N. Svilans, Electron. Lett. (16), 602, 1980.
Heinz Beneking, N. Grote, and M. N. Svilans, IEEE Trans. Electron. Devices ED-28, 404, 1981.
C. Y. Chen, A. Y. Cho, P. A. Garbinski, C. G. Bethea, and B. F. Levine, “Modulated barrier photodiode: A new majority-carrier photodetector”, Applied Physics Letters, 39(4), 340–342, 1981.
SEMICONDUCTORS AND SEMIMETALS edited by R. K. Willardson & Albert C. Beer; Volume 22 Lightwave Communication Technology edited by W. T. Tsang; Part D Photodetectors Academic Press, 1985; Chapter 5 Phototransistors for Lightwave Communications by J. C. Campbell.
Weidong Zhou, S. Pradhan, P. Bhattacharya, W. K. Liu, D. Lubyshev, “Low-power phototransceiver arrays with vertically integrated resonant-cavity LEDs and heterostructure phototransistors”, IEEE Photonics Technology Letters, Vol. 13, Issue 11, pp. 1218–1220, Nov. 2001.
O. Qasaimeh, W. Zhou, P. Bhattacharya, D. Huffaker, D. G. Deppe, “Monolithically integrated low-power phototransceivers for optoelectronic parallel sensing and processing applications”, Journal of Lightwave Technology, Vol. 19, Issue 4, pp. 546–552, April 2001.
O. Qasaimeh, Weidong Zhou, P. Bhattacharya, D. Huffaker, D. Deppe, “Monolithically integrated low-power phototransceiver incorporating microcavity LEDs and multiquantum-well phototransistors”, IEEE Photonics Technology Letters, Vol. 12, Issue 12, pp. 1683–1685, Dec. 2000.
O. Qasaimeh, W. Zhou, P. Bhattacharya, D. Huffaker, D. Deppe, “Ultra-low power monolithically integrated InGaAs/GaAs phototransceiver incorporating a modulated barrier photodiode and a quantum dot microcavity LED”, Lasers and Electro-Optics Society 2000 Annual Meeting. LEOS 2000. 13th IEEE Annual Meeting. Vol. 1, pp. 285–286, 13–16 Nov. 2000.
S.-W. Tan, H.-R. Chen, W.-T. Chen, M.-K. Hsu, A.-H. Lin, W.-S. Lour, “Characterization and Modeling of Three-Terminal Heterojunction Phototransistors Using an InGaP Layer for Passivation”, Electron Devices, IEEE Transactions on, Volume: 52, Issue: 2, pp. 204–210, February 2005.
Der-Feng Guo, “Optoelectronic switch performance in double heterostructure emitter bipolar transistor”, Solid-State Electronics, Vol. 45, pp. 1 179–1 182, 2001.
M. Shishikura, I. Nakamura, S. Tanaka, Y. Matsuoka, T. Ono, T. Miyazaki, and S. Tsuji, “A symmetric double-core InGaAlAs waveguide photodiode for hybrid integration on optical platforms”, IEEE Lasers and Electro-Optics Society Annual Meeting, 1996. Volume 1, pp. 12–13, Nov. 1 8–19, 1996, Boston, Mass. U.S.A.
M. Shishikura, S. Tanaka, H. Nakamura, Y. Matsuoka, S. Kikuchi, K. Nagatsuma, R. Sudo, T. Miura, T. Ono, and S. Tsuji, “Highly reliable operation of InGaAlAs mesa-waveguide photodiodes in a humid ambient”, 11th International Conference on Integrated Optics and Optical Fibre Communications and 23rd European Conference on Optical Communications (Conf. Publ. No.: 448), Vol. 4, pp. 97–100, Sep. 22–25, 1997.
T. Chino, K. Matsuda, H. Adachi, J. Shibata, “Characteristics of photonic parallel memory in relation to fabrication process”, IEE Proceedings J. Optoelectronics, Vol. 138, Issue 2, pp. 128–132, April 1991.
T. Chino, H. Adachi, K. Matsuda, “A photonic parallel memory with air-bridge interconnections for large scale integration”, IEEE Photonics Technology Letters, Vol. 5, Issue 5, pp. 548–551, May 1993.
Kenichi Matsuda, Jun Shibata, “Optoelectronic integrated circuit with optical gate device and phototransistor”, U.S. Pat. No. 5,014,096, filed on Feb. 1st, 1990 and issued on May 7th, 1991.
Ying Huang, R. I. Hornsey, “Current-mode CMOS image sensor using lateral bipolar phototransistors”, IEEE Transactions on Electron Devices, Vol. 50, Issue 12, pp. 2570–2573, December 2003.
Derek L. Knee, “Photocell layout for high-speed optical navigation microchips”, U.S. Pat. No. 6,037,643, filed Feb. 17th, 1998 and issued Mar. 14th, 2000.
Jie Yao, “High-sensitivity high-resolution photodetector array”, U.S. patent provisional application No. 60/476,922 filed on Jun. 9, 2003.
With applications in military night vision, medical imaging, security and law enforcement, etc., image intensifiers amplify light, turning faint low-contrast images into bright high-contrast ones. Despite the first use of military night vision equipment during World War II, it has been a 50-year dream of the military to gain significant nighttime battlefield advantage by equipping each and every combatant with night vision goggles, the core of which is the image intensifier technology. The semiconductor image intensifier disclosed in this invention is expected to deliver such high performance as well as low cost to ultimately reach that goal.
At present, Delft Electronic Products, ITT and Northrop-Grumman make the best image intensifiers supplying the European Union and the United States, respectively. Using a hybrid of semiconductor and vacuum tube technologies, these image intensifier tubes provide adequate brightness enhancement, but suffer from limited view angle, from mechanical fragileness, from short operating lifetime and from high manufacturing cost. All these drawbacks are typical of vacuum tube technology, calling for a completely semiconductor solution with performance matching or exceeding that of the current hybrid image intensifier tubes. The U.S. Defense Advanced Research Program Agency (DARPA) has solicited such solutions for several times over the past few decades, with the latest such contract being awarded in the year of 2004. Such a semiconductor image intensifier shall also find applications in medical imaging, law enforcement and security, etc. This invention presents such a design of semiconductor image intensifier, the core component of which is the surface-passivated mesa-structure heterojunction-phototransistors (HPTs).
HPTs have been studied for potential applications in high-speed fiber-optic communication networks. The paper by J.-P. Vilcot et al. (1997) and the paper by M. C. Brain and D. R. Smith (1983) report such comprehensive studies. HPTs optimized for this application have high bandwidth, typically in the GHz (Giga-Hertz) range, but allow high dark currents, typically in the nA (nano-Ampere) range, and allow high bias currents. The high dark current and high bias current render these HPTs unsuitable for use in image intensifiers, which always require dark currents in the pA (pico-Ampere) range or lower, and which also benefit from zero bias current while maintaining gain well above 1,000. Consequently, HPTs optimized for fiber-optic communications are typically not adequate for image intensifiers.
H. Beneking et al. reported one of the earliest attempts to use HPT in GaAs material system for image intensification with limited success. Their work was carried out at a time when semiconductor bulk material quality was still improving, and when surface problems and surface passivation have yet to be explored. Beneking et al. built a semiconductor image intensifier with phototransistors of planar structures. The entire image intensifier device was successful as optical amplifier and as an infrared-to-visible wavelength converter at least for normal light levels. Planar structures do not have mesas or isolation trenches, thus completely eliminating mesa sidewalls and their passivation. However, the tradeoff is large crosstalk over lateral distances of 25 to 100 microns, mainly because of carrier diffusion. This crosstalk in planar HPTs smears out image points 25 microns or closer. This invention solves the crosstalk problem with passivated mesa HPTs defined by their surrounding isolation trenches.
During the same years, C. Y. Chen et al. proposed a novel modulated-barrier-photodiode. Just as reviewed by J. C. Campbell in SEMICONDUCTORS AND SEMIMETALS, it is a phototransistor with base width pushed to the extreme. In both literatures, two groups of authors independently experimented with phototransistors. Gain exceeded 1000 in GaAs at very low light levels of 1 nW with mesa structures larger than 100 microns in lateral dimensions. Neither surface issues nor surface passivation techniques have been mentioned. Mesa sidewalls, if not passivated, will lead to high dark current and significantly reduced small signal gain in HPTs with lateral dimensions smaller than 25 microns, the carrier diffusion length. This invention teaches that mesa isolation trenches solve the inter-pixel crosstalk problem, and that mesa sidewall passivation leads to high small signal gain and low dark current.
The latest papers by P. Bhattacharya et al. report no sidewall passivation of their HPTs, thus limiting their dark current to 1 nA or higher, and limiting their HPT size to 30 microns or larger. It is worth noting that S. W. Tan et al. used the word passivation for ledge passivation, a very successful standard technique widely used in the HBT electronics industry, for their mesa HPTs 150 microns in lateral size. No sidewall surface passivation was mentioned at all. In order to reduce HPT mesa size to well below 25 microns and facilitate large-scale integration of mesa HPTs, the passivation of mesa sidewalls is crucial. Successful passivation of HPT mesa sidewalls prevents small signal gain reduction as well as reduces dark current.
Silicon nitride has been reported by M. Shishikura et al. to passivate p-i-n photodiodes in the InP material system with high reliability. For a simple p-i-n photodiode, the passivation of sidewall surface serves only to reduce dark current. No gain mechanism is involved. This invention, in contrast, teaches several different inorganic passivation materials and techniques aimed at achieving high gain for HPT as well as reducing dark current to pA or below.
Organic passivation materials, for example, result in >20,000 nA of bias current and >10 nA of dark current for bi-stable switch digital memory devices reported by T. Chino et al. (1991). While these might be great results for digital switches, they are certainly totally unacceptable for image intensifier applications. Hence this invention does not cover organic passivation materials at all.
Years later, T. Chino et al. (1993) attributed their lowest holding current of 6,000 nA to unintentionally and atmospherically formed indium oxide (In2O3) on the exposed surfaces of InP-based semiconductors, essentially eliminating the need for surface passivation for the digital logic large-signal (micro-Watt) operation of the HPT-LED pair. However, In2O3 is typically not an insulator but a conductor, shorting instead of passivating their HPT sidewalls, and leading to unacceptably high currents for image intensifier applications with input analog optical signals at pico-Watt or lower optical power. This invention insists the passivation of HPT sidewall with an electrical insulator.
With sidewall-passivated mesa HPTs described in this invention, one can simultaneously achieve pixel sizes well below 10 microns, trench isolation, amplifier gain well above 1,000 at low illumination levels, and dark current in the pA range or below.
Other device structures not covered by this invention are possible for imaging. Ying Huang et al. reported one such example, where the planar device is readily compatible with CMOS technology, but has a low fill factor for optical absorption. In contrast, the mesa HPTs described in this invention provide a means of vertically integrating amplification function with photodetection function, maximizing fill factor.
The preferred layout of this invention requires the HBT amplifier not to be at equal distance from the geometric centers of neighboring HPTs, opposite to the teaching in U.S. Pat. No. 6,037,643 by Derek L. Knee.
In a preferred embodiment of this invention, the semiconductor image intensifier is composed of two semiconductor chips flip-chip bonded into one device, the first chip being the GaAs-based HPT or photo-Darlington array, and the second chip being the GaN-based LED array.
It is worth noting that the high-gain low-noise amplification is performed in the electrical domain with a current amplifier, which is preceded by the optical-to-electrical conversion at the photo-detector and followed by the reverse electrical-to-optical conversion at the LED. Can we use direct optical-to-optical amplification instead? While we have numerous mature mass-produced devices and circuits capable of high-gain low-noise amplification in the pure electrical domain, the current electro-optical technologies do not allow efficient high-gain amplification in the pure optical domain. With the availability of highly efficient photo-detectors, electrical amplifiers and LEDs, we choose the much more practical path of electrical amplification for our image intensifier.
One of the critical components of the semiconductor image intensifier described in this invention is the optical isolation layer. Without the optical isolation materials between the LED-based display array and the HPT-based photodetector array, the output analog signal from the LED will partially enter the HPT, forming in a positive-feedback loop, which can result in undesirable strong non-linearity in the transfer function (from input to output) of the image intensifier.
One example of such non-linearity is the switching bi-stability reported by T. Chino et al. in their HPT-LED pair, which is designed for switching and intended for use in digital photonic parallel memory devices. Switching, however, is absolutely intolerable in the analog-signal image intensifier. Also, the image intensifier disclosed in this invention operates at low input light levels in the pico-Watt (10^-12 Watt) range per pixel, while the digital memory devices reported by T. Chino et al. operate at a minimum of 1 micro-Watt (10^-6 Watt) of optical power generated by the 10 micro-Amperes of holding current through the LED. In fact, image intensifiers are optimized for the low-light limit, while digital memory avoid low signal levels due to the difficulties associated with noise, crosstalk, compatibility with other circuits, etc. As a result, no surface passivation is needed for the digital logic large-signal (micro-Watt) operation of the HPT-LED pair reported by T. Chino et al., whereas excellent surface passivation is absolutely indispensable for the analog small-signal (pico-Watt) operation of the image intensifier described in this invention. Unintentionally and atmospherically formed on the exposed surfaces of InP-based semiconductors as reported by T. Chino et al., indium oxide (In2O3) is typically not an insulator but a conductor, shorting instead of passivating their HPT sidewalls.
Having examined device performance, we now go on to the device design and device structure. FIG. 2 of the 1991 paper by T. Chino et al. shows the working mechanism of their HPT-LED memory device. For the off state of their HPT-LED switch to be stable, the slope dl/dPFB of the current I versus optical feedback power PFB curve (the l-PFB curve) in the low current and low optical power region has to be larger for the LED than for the HPT. If anyone were to use this region for an image intensifier, however, one would get reduction, instead of intensification, of image brightness even with complete suppression of optical feedback from the LED to the HPT. In contrast, the opposite is required of an image intensifier, which typically delivers the largest gain and amplification in the low optical power region. In fact, the above-mentioned characteristics of dl/dPFB being larger in LED than in HPT is one important result of the damaged mesa sidewalls both for the LED and for the HPT. With ideal damage-free mesa sidewalls, dl/dPFB of the HPT is almost always a lot higher than dl/dPFB of LED. Damages in the mesa sidewalls of an LED increase its dl/dPFB, while damages in the mesa sidewalls of an HPT decreases its dl/dPFB. That is why T. Chino et al. wrote in paragraph 3.3 of their 1991 paper that their “switches need plasma damage to the sidewalls of mesa to some degree to show bistability”. None of their switches formed by wet chemical etching exhibited bistability, because these switches were “near the ideal surface condition” (paragraph 3.2 of their 1991 paper). The surface processing techniques reported by T. Chino et al. result in a mesa sidewall with more damages than the wet-etched case, with holding currents in the 10,000 nA range and dark leakage currents in the 10 nA range. In contrast, this invention teaches the opposite. In order to achieve dark leakage currents in the pA (pico-Ampere, namely 1/1000 nA) range or lower, we passivate the mesa sidewalls to reduce the damage on the typically wet-etched sidewalls, resulting in mesa sidewall surfaces of higher quality than the purely wet-etched case. This invention maintains high dl/dP for the HPT and high dP/dl for the LED, and hence, with the suppression of optical feedback, makes the image intensifier optimized for low input optical power.
The HPT combines the functionality of a p-i-n photodiode with that of a HBT. Hence the HPT is neither simply a p-i-n photodiode nor simply an HBT. In fact, the HPT disclosed in this invention is optimized very differently from that of an HBT. An HBT is definitely always a three-terminal device with contacts to the base layer, while the HPT disclosed in this invention is preferred to be a two-terminal device with floating base. A high-speed HBT typically has heavy base doping levels of 10^(+19)/cm^3 or higher for the reduction of base resistance, while the HPT disclosed in this invention is preferred to have low base doping of 2*10^(+17)/cm^3. HBTs almost never need to operate at base currents as low as pico-Amperes in a 50-Ohm microwave system, while the HPT disclosed in this invention typically operates at pico-Amperes of equivalent base current if not lower. Consequently, the passivation target and the passivation techniques are also very different. The most effective surface passivation technique for an HBT is ledge passivation on top of the base layer, as studied by numerous investigators including S. W. Tan et al. and Der-Feng Guo, while the HPT disclosed in this invention employs sidewall passivation using inorganic insulators. The sidewall passivation according to this invention simultaneously suppresses dark current and maintains high current gain. In the preferred embodiment of this invention, there is no place for ledge passivation in the two-terminal HPT at all. In short, HPTs and HBTs are optimized very differently, without too many optimization techniques in common.
This invention describes the optimization techniques for a mesa HPT array uniquely developed for image intensifier applications.
This invention was first filed on Jun. 9, 2003 as U.S. patent provisional application No. 60/476,922 under the title of “High-sensitivity high-resolution photodetector array”, wherein more details are available.